The present invention relates to metal billets, slabs, plates, rods, and sputter targets and other metal articles. More particularly, the present invention relates to a method of producing a metal preferably having a uniform fine grain size, a homogeneous microstructure, low texture banding, and/or an absence of surface marbleizing that is useful in making sputter targets and other objects.
Certain observable properties of sputter targets and sputter target materials are desirable for enhancing the sputtering performance of valve metal sputter targets (See, e.g., Michaluk, “Correlating Discrete Orientation and Grain Size to the Sputter Deposition Properties of Tantalum,” JEM, January, 2000; Michaluk, Smathers, and Field, Twelfth International Conference on Texture of Materials, J. A. Szpunar (ed.), National Research Council of Canada, 1999, p. 1357). Fine grain size and homogeneous microstructure that is substantially free of sharp texture bands are examples of such properties. Grain size, grain uniformity, and textural homogeneity of metal material, generally, and of target material in particular, are measurable qualities, by methods described, for example, in U.S. Pat. No. 6,462,339 B1 (Michaluk et al.), and Wright et al., “Scalar Measures of Texture Heterogeneity,” MATERIAL SCIENCE FORUM, Vols. 495-497 (Sept. 2005) pp. 207-212, all incorporated herein in their entirety by reference.
Hence, an ongoing interest exists in relevant markets to develop processes for producing high purity metal articles, like sputter targets having the above-described metallurgical and textural qualities. Conventional metalworking multi-step sequences incorporating forging and/or rolling steps, combined with one or more intermediate annealing steps as well as one or more cleaning steps, are typically used in manufacturing suitable mill forms and are generally described by C. Pokross, “Controlling the Texture of Tantalum Plate,” JOURNAL OF METALS, October 1989, pp. 46-49; and J. B. Clark, R. K. Garrett, Jr., T. L. Jungling, R.I. Asfahani, “Influence of Transverse Rolling on the Microstructural and Textural Development in Pure Tantalum,” METALLURGICAL TRANSACTIONS A, 23A, pp. 2183-91 (1992), which are incorporated herein in their entirety by reference. An example of a multi-step forging, cleaning, annealing, and rolling process to produce a tantalum sputter target having fine grain size and a homogeneous texture is described in U.S. Pat. No. 6,348,113 (Michaluk et al.), incorporated herein in its entirety by reference.
Tantalum has emerged as the primary diffusion barrier material for copper interconnects employed in advanced integrated circuit microelectronic devices. During the fabrication sequence of such microelectronic devices, tantalum or tantalum-nitride barrier films are deposited by physical vapor deposition (PVD), a well-established process whereby a source material (termed a “sputtering target”) is eroded by high-energy plasma. Bombardment and penetration of plasma ions into the lattice of the sputtering target causes atoms to be ejected from the surface of the sputtering target which then deposit atop the substrate. The quality of sputter-deposited films is affected by many factors, including the chemistry and metallurgical homogeneity of the sputtering target.
In recent years, research efforts have focused on developing processes to increase the purity, reduce the grain size, and control the texture of tantalum sputtering target materials. For example, U.S. Pat. No. 6,348,113 (Michaluk et al.) and U.S. Patent Application Nos. 2002/0157736 (Michaluk) and 2003/0019746 (Ford et al.), each of which is incorporated herein by reference, describe metalworking processes for attaining select grain sizes and/or preferred orientations in tantalum materials or tantalum sputtering target components through particular combinations of deformation and annealing operations.
A method suitable for producing large lots and bulk quantities of high purity tantalum sputtering targets having microstructural and textural homogeneity is described in U.S. Pat. No. 6,348,113 (Michaluk et al.). While high volume manufacturing processes offer significant cost benefits compared to batch processes, they often cannot achieve tight dimensional tolerances by means of a standardized and repeatable deformation sequence. The mechanical responsiveness of high purity tantalum ingots and heavy rolling slabs is highly variable due to their large, inhomogeneous grain structure. Imposing a predefined and consistent rolling reduction schedule on heavy slabs of high purity tantalum can result in a divergence in plate thickness with each reduction pass, and ultimately would yield plate products having an excessive variation in gauge. Because of this behavior, conventional methods for rolling tantalum plate from heavy slab is to reduce the mill roll gap by a certain amount depending on the width and gauge of the plate, then adding light finishing passes to achieve gauge tolerances typically about +/−10% of the target thickness.
Some rolling theory prescribes that heavy reductions per rolling pass are necessary to achieve a uniform distribution of strain throughout the thickness of the component, which is beneficial for attaining a homogeneous annealing response and a fine, uniform microstructure in the finished plate. Scale presents a primary factor that hinders the ability to take heavy rolling reduction when processing high volume tantalum slabs to plate since heavy reduction (e.g., true strain reduction) may represent more of a bite than the rolling mill can handle. This is especially true at the commencement of rolling where the slab or plate thickness is largest. For example, a 0.2 true strain reduction of a 4″ thick slab requires a 0.725″ reduction pass. The separating force that would be necessary to take such a heavy bite would exceed the capability of conventional production rolling mills. Conversely, a 0.2 true strain reduction on a 0.40″ thick plate equates to only a 0.073″ roll reduction, which is well within the capabilities of many manufacturing mills. A second factor that affects the rolling reduction rate of tantalum is the plate width. For a given roll gap per pass, plate gauge, and mill, wider plates will experience a smaller amount of reduction per rolling pass than narrow plates.
Since the processing of bulk tantalum cannot rely solely on heavy rolling reductions to reduce slab to plate, strain is not likely to be uniformly distributed throughout the thickness of the plate. As a result, the product does not evenly respond to annealing, as evidenced by the existence of microstructural and textural discontinuities in tantalum plate as reported in the literature (e.g., Michaluk et al. “Correlating Discrete Orientation and Grain Size to the Sputter Deposition Properties of Tantalum,” JEM, January, 2002; Michaluk et al., “Tantalum 101: The Economics and Technology of Tantalum,” Semiconductor Inter., July, 2000, both of which are incorporated herein by reference). The metallurgical and textural homogeneity of annealed tantalum plate is enhanced by incorporating intermediate anneal operations to the process as taught by U.S. Pat. No. 6,348,113. However, incorporating one or more intermediate annealing operations during the processing of tantalum plate will also reduce the total strain that is imparted to the final product. This, in turn, would lessen the annealing response of the plate, and hence limit the ability to attain a fine average grain size in the tantalum product.
The existence or occurrence of a marbleized structure in tantalum has been deemed to be detrimental to the performance and reliability of tantalum sputtering target material and components. It has only recently been discovered by the inventors that two distinct types of marbleizing can be found in tantalum and other metals: marbleizing observed along the sputtered surface of an eroded tantalum target or component, and marbleizing observed about the as-fabricated surface of the tantalum target or component. In an eroded tantalum sputtering target, marbleizing is formed from the mixture of exposed, sputter-resistant (100) texture bands (that appear as lustrous regions) about the matte finish of the matrix material (created by multi-facet sputter-eroded grains). The propensity for marbling of a sputter-eroded surface is minimized by or eliminated in tantalum sputtering targets or components that are processed to have a homogeneous texture through the thickness of the tantalum target, as described in U.S. Pat. No. 6,348,113. An analytical method for quantifying the texture homogeneity of tantalum sputtering target materials and components is described in U.S. Pat. No. 6,462,339 (Michaluk et al.), which is incorporated herein by reference. Another analytical method for quantifying banding is described in U.S. Patent Application No. 60/545,617 filed Feb. 18, 2004 and is incorporated herein by reference.
Surface marbling can be resolved along the as-fabricated surface of wrought tantalum materials or sputtering components after light sputtering (e.g., burn-through trials) or by chemical etching in solutions containing hydrofluoric acid, concentrated alkylides, or fuming sulfuric and/or sulfuric acid, or other suitable etching solutions. In annealed tantalum plate, surface marbleizing appears as large, isolated patches and/or a network of discolored regions atop the acid cleaned, as-rolled surface. The marbleized surface of tantalum can be removed by milling or etching about 0.025″ of material from each surface; however, this approach for eliminating surface marbling is economically undesirable. Surface marbling can be considered regions that have different average grain size in the regions and/or regions of differing primary texture (e.g., (100) vs. (111)). Surface marbling can be more due to grain size variances, wherein the regions can have a variance in average grain size of ±2 ASTM or more, such as ±2 ASTM to ±5 ASTM, or ±2 ASTM to ±4 ASTM, or ±2 ASTM to ±3 ASTM, when comparing the average grain size in one region to the average grain size in another region.
Accordingly, a need exists for a method to produce a sputter target material having superior metallurgical and textural qualities, and to reduce the costs associated with production of sputter targets exhibiting such qualities.